Apparatus of this type are known in the art. Generally, the purpose of these apparatus is to heat products, and particularly food products, such that bacteria can be killed so as to achieve a prolonged shelf time of the product. Thereby, it is important that quality features, such as color, taste, consistency and texture do not significantly change. This is particularly important with fruit or fruit pieces containing food products as the structure of the fruits may readily be destroyed by heat. Therefore, the challenge resides in the fact that processing methods and apparatus have to be developed with which the desired characteristics of the products may be maintained as much as possible.
An apparatus for heating and sterilizing of products streams is known, for example from U.S. Pat. No. 6,246,040 B1. In the apparatus described therein, product streams moving in glass or Teflon tubes are electrically heated by strong electric alternating fields. To this end, radio frequency fields are applied to electrodes located outside of the tubes. Polar, i.e. electrically asymmetric, molecules contained in the product to be heated are subjected to a motion owing to their tendency to adjust to the electric field. Hence, the motion of the molecules produces heat. Hereinafter, this method of heating will be referred to as radio frequency heating. Such an apparatus enables a rapid and homogeneous heating even in non-homogeneous product mixtures. In these systems, a large portion of the energy, however, is lost in the walls of the tubes so that the efficiency of the apparatus is low. Moreover, due to the presence of high voltages, which may be generated by means of complex power electronics only, there exists the risk of spark breakthrough. For these reasons, the productivity of apparatus of this type is low and hence, the production costs are correspondingly high.
In another well known apparatus, a method of ohmic heating is used. Hereby, electric current flows through the medium to be heated. To this end, low-frequency voltage sources (up to 60 Hz) are used. The electrodes directly contact the product so that energy losses are maintained at very low levels. However, this method may successfully be used with homogeneous product streams only. Since, the product channels have to be relatively narrow (about 1 cm), this method may not easily be usable with fluids enriched with fruit pieces and the flow rates achieved are low. Moreover, there is a risk of electrolysis, thereby producing hydrogen. This results in a relatively short lifetime of the electrodes which are mostly made of carbon.